Primary air is utilized with pulverized coal burners for preheating the pulverized coal and thereby improving ignition performance of the coal. This is especially important in hard-to-ignite coal. U.S. Pat. No. 4,448,135 to Dougan et al. and assigned to the Babcock and Wilcox Company, discloses an in-line coal air separator which improves low load operation by separating the air entrained with pulverized coal into a rich-coal stream and into a lean-coal moisture-laden stream.
U.S. Pat. Nos. 4,173,189 to Cooper and 4,381,718 to Carver et al. both disclose a boiler system wherein the combustion air is preheated. U.S. Pat. No. 4,412,496 to Trozzi relates to a boiler system wherein the air-coal stream is split into separate streams. U.S. Pat. No. 4,492,171 to Brashears et al. discloses a solid fuel burner wherein the fuel is mixed with combustion air prior to being burned. U.S. Pat. No. 4,515,094 to Azuhata et al. discloses a burner having primary and secondary nozzles for jetting into the combustion chamber a fuel stream having a particular ratio. These references are all drawn to efforts in improving the operation and efficiency of solid fuel burners.
While it is known that the delivery of hotter primary air to the burner will significantly improve the ignition performance of different fuels and especially that of low volatile matter coals which are notoriously difficult to ignite, generally temperatures of only up to 200.degree. F. are possible. Primary air leaving a pulverizer usually has a temperature of appproximately 150.degree. to 175.degree. F. as coal volatile matter drops, and this temperature can be as high as 200.degree. F. for low volatile coal. Further increases would be beneficial but are limited by the temperature of the primary air available to the pulverizer mill, and by the mechanical design of the mill which generally has a maximum allowable mill outlet temperature of 200.degree. F.
An alternate approach would be to use a bin system which uses a "fresh" primary air stream to transport coal from the bin to the burners. Primary air streams for transporting such coal may for example range from 500.degree. to 600.degree. F. This would greatly improve the ignition performance of very low volatile coal. Several problems exist however, when using a bin system. Such systems generally pneumatically transport the coal from a pulverizing mill to a bin after which this air is vented. The air that is then used to transport the coal from the bin to the burners is heated and often is hotter than that achievable when the same air is used to convey the pulverized coal directly from the mill to the burners. This is because the limitations of the mill are by-passed. However, bin systems are essentially never used in modern plants due to the added expense and the potential explosion hazards associated with stored pulverized coal. These expenses are significant due to the use of air/coal separation equipment, storage bins, controls, inerting equipment and the like. Bin systems also have the disadvantage of difficulties in metering the coal flow. For this reason a primary air exchange system is preferable over a bin system.
It is also advantageous to improve ignition characteristics over those available in conventional systems. Burners with poor ignition performance on difficult fuels burn large quantities of oil or natural gas to maintain fuel stability. This is a poor use of a precious resource and expensive as these auxiliary fuels are two or three times more costly than coal on a BTU basis. Therefore, incremental cost increase for improved burner performance is easily justified.
Another means for firing different fuels in conventional burners is by resorting to a special furnace design. Low volatile coals and anthacites are usually fired in a downshot "W" furnace, with the lower furnace refractory lined. This arrangement relies on a hot furnace and additional residence time to ignite and burn out these coals. Such a furnace design is effective but considerably more expensive than conventional wall-fired designs. A primary air exchange burner permits the use of conventional furnace designs for a much broader range of difficult-to-ignite fuels.
Accordingly, an object of the present invention is to improve pulverized coal ignition while avoiding a reduction in efficiency of the burner. Another object of the invention is to provide a primary air exchange for a pulverized coal burner which is simple in design, rugged in construction and economical to manufacture. A further object of this invention is to remove a portion of the primary air from the coal/air mixture prior to combustion and substitute this removed air with heated air whose quantity is determined by the ignition requirements of the to-be-burned coal.